Resonator device

ABSTRACT

A resonator device for measuring stress comprises at least two resonators, each resonator comprising an inter-digitated transducer structure arranged between two reflecting structures on or in a piezoelectric substrate, characterized in that the at least two resonators are arranged and positioned such that they have two different wave propagation directions, and each resonator comprises at least two parts with the area between the two parts of the at least two resonators forming a cavity, wherein the cavity is shared by the at least two resonators. A differential sensing device may comprise at least one resonator device as described herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/EP2020/057616, filed Mar. 19, 2020,designating the United States of America and published as InternationalPatent Publication WO 2020/200810 A1 on Oct. 8, 2020, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. FR1903408, filed Mar. 29, 2019.

TECHNICAL FIELD

The present disclosure relates to an acoustic wave device for sensorapplications, and more particularly to acoustic wave differentialsensors.

BACKGROUND

Sensors are of growing importance and become more and more ubiquitous inevery-day life. Microelectromechanical systems (MEMS) are an attractiveoption to answer the demand for increased performance of sensors alongwith decreased sizes and costs. Surface acoustic wave (SAW) sensors, andto a lower extent bulk acoustic wave (BAW) sensors or Lamb wave or Lovewave or shear-plate mode acoustic sensors, offer particularlyadvantageous options due to a wide variety of measurable ambientparameters including temperature, pressure, strain and torque, forexample.

Acoustic wave sensors utilize the piezoelectric effect to transduce anelectrical signal into a mechanical/acoustic wave. SAW-based sensors arebuilt on single-crystal piezoelectric materials like quartz (SiO₂),lithium niobate (LiNbO3), lithium tantalate (LiTaO3), langasite (LGS)and aluminum nitride (AlN) or zinc oxide (ZnO) deposited on silicon. Aninter-digitated transducer (IDT) converts the electrical energy of anincident electrical signal into acoustic wave energy. The acoustic wavetravels across the surface (or bulk) of a device substrate via theso-called delay line to another IDT that converts the acoustic wave backto an electrical signal that can be detected. In some devices,mechanical absorbers and/or reflectors are provided in order to preventinterference patterns and reduce insertion loss. In some devices, theother (output) IDT is replaced by a reflector that reflects thegenerated acoustic wave back to the (input) IDT that can be coupled toan antenna for remote interrogation of the sensor device.

A particular class of acoustic sensors comprises resonators exhibitingresonator frequencies that vary according to varying ambient conditions.A conventional surface wave resonator, for example, comprises anelectroacoustic transducer with inter-digitated combs arranged betweenBragg mirrors. At the resonance frequency, the condition of synchronismbetween the reflectors is satisfied making it possible to obtain acoherent addition of the different reflections, which occur under thereflectors. A maximum of acoustic energy is then observed within theresonant cavity and, from an electrical point of view, a maximum ofamplitude of the current admitted by the transducer is observed.

Differential acoustic wave sensors comprise two or more resonatorsexhibiting different resonance frequencies wherein differences in themeasured frequencies reflect variations in the parameter to be measuredas, for example, strain.

The differential sensor must be able to segregate the origin of theperturbation and to reduce or suppress contributions from otherparameters, such as contributions from vibrations or temperaturefluctuations. This requires the development of a differential sensor forwhich temperature and vibration sensitivity must be as small as possibleor rigorously equal from one resonator to another to allow for rejectionby signal subtraction.

FIG. 1 shows such a surface acoustic wave differential sensor accordingto the state of the art. This sensor is configured to measure stress,e.g., on a rotating object. The surface acoustic wave differentialsensor 100 comprises two surface acoustic wave resonators 102, 104provided on a piezoelectric substrate 106. Each surface acoustic waveresonator 102, 104 comprises an inter-digitated transducer structure108, 110 and a pair of reflecting structures 112, 114, 116, 118. Thereflecting structures 112, 114 are arranged on each side of theinter-digitated transducer structure 108 and the reflecting structures116 and 118 on each side of the interdigitated transducer structure 110,in both cases with respect to the direction of propagation of theacoustic wave, see arrows 120, 122, of the corresponding transducerstructure 108, 110. The two resonators 102 and 104 are electricallyconnected to each other in a differential way by two conductive lines124 and 126.

Both resonators 102, 104 are positioned on the piezoelectric substrate106 with an angle Ψ of ±45° in regards with the crystallographic axis Xof a singly rotated Quartz substrate 106, corresponding to the usualpropagation direction of a Rayleigh wave. Thus, the two resonators areperpendicular to each other.

Each resonator 102, 104 exhibits a resonance peak at a frequency f1, f2,respectively.

The resonators 102, 104 are connected in parallel and then connected toan antenna to be wirelessly interrogated, the differential measureresulting for the difference of the resonance frequencies measuredeither simultaneously or sequentially.

By aligning one resonator 102, 104 in parallel with the radial directionof a rotating object, the differential sensor 100 is sensitive to radialstress occurring on the object. On the occurrence of radial stress,deformations occur in the sensor leading to extension in the oneresonator and contraction in the other. This leads to changes withopposite signs and typically the same absolute value, in the resonantfrequencies. Thus, the difference in the resonant frequencies changes bythe sum of the two absolute values. By measuring the variation of thedifference Δf between the two resonant frequencies, one can determinethe applied force, as the difference Δf is linearly proportional to thetorque M. Unwanted temperature variation effects, do, however, cancelout, as they will affect both resonators in the same way.

However, in the differential sensor 100 according to the state of theart, the stress state is not measured at the same location by the tworesonators 102, 104, nor the temperature. Consequently, the measurementmight be negatively affected by inhomogeneities in the material of theobject, leading to errors in the stress determination.

The object of the present disclosure is therefore to overcome thedrawback cited above resulting in an improved sensing device.

BRIEF SUMMARY

The object of the present disclosure is achieved by a resonator devicecomprising at least two resonators, each resonator comprising aninter-digitated transducer structure arranged between two reflectingstructures on or in a piezoelectric substrate, characterized in that theat least two resonators are arranged and positioned such that they havetwo different wave propagation directions, and each resonator comprisesat least two parts with the area between the two parts of the at leasttwo resonators forming a cavity, wherein the cavity is shared by the atleast two resonators. Thus, the two resonators of the device measure atthe same location and the measurement is therefore less influenced byinhomogeneities in the material on which the resonators are attached.This is contrary to the state of the art device, where each resonatormeasures at a different location.

According to a variant of the present disclosure, each of the at leasttwo parts of the at least two resonators can comprise at least onereflecting structure and a part of the inter-digitated transducerstructure of the corresponding resonator. The device as describedenables management of parasites due to directivity effects.

According to a variant of the present disclosure, the inter-digitatedtransducer structure of the resonator can comprise inter-digitated combelectrodes, and wherein for at least one transducer structure of the atleast two resonators, the inter-digitated comb electrodes are defined bythe Bragg condition given by p=λ/2, λ being the operating acousticwavelength of the transducer structure and p being the electrode pitchof the transducer structure. The device as described enables managementof parasites due to directivity effects.

According to a variant, the two different wave propagation directions ofthe at least two resonators can form an angle Θ with each other, Θ beingequal to ±900 or smaller.

According to a variant of the present disclosure, the electrodes of theinter-digitated transducer structure of the one resonator can beelectrically connected with the electrodes of the transducer structureof the other resonator in a differential way. The connection between theelectrodes of the at least two resonators can be either in parallel, orin series, depending on their operating conditions. Thus, the deviceaccording to the present disclosure can operate either on resonance oranti-resonance, depending on the design choices, in contrast to thestate of the art device.

According to a variant of the present disclosure, at least one of theresonators can be arranged and positioned such that its wave propagationdirection is parallel to one of the crystalline axis of thepiezoelectric substrate.

According to a variant of the present disclosure, at least one of theresonators can be arranged and positioned such that its wave propagationdirection makes an angle Ψ to one of the crystalline axis of thepiezoelectric substrate, in particular, an angle Ψ equal to ±45°.

According to a variant of the present disclosure, at least a part of thesurface of the cavity can be metalized. The device as described enablesfiltering or selection of the possible modes of the structure, and evenallows for operating in a coupled mode configuration.

According to a variant of the present disclosure, the metallization ofthe cavity can comprise at least one or more grating. When more than onegrating is present, the gratings are superimposed to each other. Thedevice as described enables filtering or selection of the possible modesof the structure, and even allows for operating in a coupled modeconfiguration.

According to a variant of the present disclosure, each one of thereflecting structures of the resonators can comprise one or moremetallic strips, the metallic strips being connected to each other orconnected to ground. Thus, the resonators can also be tag devices.Furthermore, the connection of the metallic strips to each other or toground results in an improvement of the reflection coefficient of thereflecting structures at the Bragg condition. At the Bragg condition,the reflected waves due to electrical and mechanical loading are inphase so that an improved reflection coefficient of the reflector at theBragg condition results in a better detection of the reflected waves bythe corresponding transducer structure.

According to a variant of the present disclosure, the resonator can be asurface acoustic wave resonator (SAW), a bulk acoustic wave resonator(BAW), a Lamb wave, a Love wave or shear-plate mode acoustic resonator.

The object of the present disclosure is also achieved by a differentialsensing device, which may comprise at least one resonator device asdescribed previously. The differential sensing device enablesmeasurement of both the radial and tangential forces in a differentialmanner, namely the sensor system enables measurement of the stress bysegregating the origin of the perturbation and immunity to other stimulisuch as temperature, vibrations or pressure.

According to a variant of the present disclosure, the propagationdirection of one of the resonators can be parallel or perpendicular to aradial direction to sense a radial force. The resonator enablesmeasurement of the radial forces in a differential manner, namely thesensing device enables measurement of the stress by segregating theorigin of the perturbation and immunity to other stimuli such astemperature, vibrations or pressure.

According to a variant of the present disclosure, the propagationdirection of one of the resonators is at an angle Ψ, in particular, at45° with respect to a radial direction to sense a tangential force. Theresonator enables measurement of the tangential forces in a differentialmanner, namely the sensing device enables measurement of the stress bysegregating the origin of the perturbation and immunity to other stimulisuch as temperature, vibrations or pressure.

According to a variant of the present disclosure, one resonator devicecan be arranged so that that its wave propagation direction is parallelto one of the crystalline axis of the piezoelectric substrate and oneresonator device can be arranged so that its wave propagation directionmakes an angle Ψ to one of the crystalline axis of the piezoelectricsubstrate, in particular, an angle Ψ equal to ±45°. The differentialsensing device enables measurement of both the radial and tangentialforces in a differential manner, namely the sensing device enablesmeasurement of the stress by segregating the origin of the perturbationand immunity to other stimuli such as temperature, vibrations orpressure.

According to a variant of the present disclosure, the differentialsensing device can further comprise an antenna connected to the at leastone resonator device.

According to a variant of the present disclosure, at least twodifferential resonator devices can be provided on the same piezoelectricsubstrate. Therefore, the fabrication process will be simpler and fastercompared to the state of the art device for which each differentialsensor is fabricated on a separate substrate, as both differentialsensors share the same structural characteristics and dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be understood by reference to the followingdescription taken in conjunction with the accompanying figures, in whichreference numerals identify features of the present disclosure.

FIG. 1 shows a resonator device according to the state of the art.

FIG. 2a shows a resonator device according to a first embodiment of thepresent disclosure.

FIG. 2b shows a resonator device according to a second embodiment of thepresent disclosure.

FIG. 3a shows the third embodiment of the present disclosure based onthe resonator device according to the first embodiment.

FIG. 3b shows the fourth embodiment of the present disclosure based onthe resonator device according to the second embodiment.

FIG. 3c shows the electrical admittance simulation of a resonator deviceaccording to the first to fourth embodiment of the present disclosure.

FIG. 4a shows the sensing device according to a fifth embodiment of thepresent disclosure.

FIG. 4b shows the electrical measurements of the sensing deviceaccording to the fifth embodiment of the present disclosure.

FIG. 5a shows the resonator device according to a first variant of thefirst and second embodiments of the present disclosure.

FIG. 5b shows the resonator device according to a second variant of thefirst and second embodiments of the present disclosure.

FIG. 5c shows the resonator device according to a third variant of thefirst and second embodiments of the present disclosure.

FIG. 5d shows the resonator device according to a fourth variant of thefirst and second embodiments of the present disclosure.

FIG. 6a shows the resonator device according to a fifth variant of thefirst and second embodiments of the present disclosure.

FIG. 6b shows the resonator device according to a sixth variant of thefirst and second embodiments of the present disclosure.

FIG. 7a shows the resonator device according to a seventh variant of thefirst and second embodiments of the present disclosure.

FIG. 7b shows the resonator device according to an eight variant of thefirst and second embodiments of the present disclosure.

FIG. 7c shows the resonator device according to a ninth variant of thefirst and second embodiments of the present disclosure.

FIG. 7d shows the resonator device according to a tenth variant of thefirst and second embodiments of the present disclosure.

FIG. 7e shows the resonator device according to an eleventh variant ofthe first and second embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure will now be described in more detail usingadvantageous embodiments in an exemplary manner and with reference tothe drawings. The described embodiments are merely possibleconfigurations and it should be kept in mind that the individualcharacteristics as described above can be provided independently of oneanother or can be omitted altogether during the implementation ofembodiments of the present disclosure.

FIG. 2a shows a resonator device according to a first embodiment of thepresent disclosure. In the following, the resonator device will bedescribed as a surface acoustic wave resonator device (SAW). Accordingto variants, bulk acoustic wave (BAW) resonators, Lamb wave or Love waveor shear-plate mode resonators could be used in the same way accordingto the present disclosure.

In FIG. 2a , the surface acoustic wave sensor 200 comprises two surfaceacoustic wave resonators 202, 204 provided over or in a surface acousticwave propagating substrate 206. Each surface acoustic wave resonator202, 204 comprises an inter-digitated transducer structure 208 a, 208 band 210 a, 210 b, each sandwiched by a couple of reflecting structures212, 214 and 216, 218. The reflecting structures 212, 214, 216, 218comprise a reflector with one or more metallic strips 220, and areconfigured to reflect the surface acoustic wave generated by theinter-digitated transducer structures.

Here, the reflecting structures 212, 214, 216, 218 are arranged with agap from the inter-digitated transducer structures 208 a, 208 b, 210 a,210 b. In a variant of the present disclosure, no gap can be presentbetween the reflecting structures and the transducer structure, so thatthe reflecting structure can be considered as continuing theinter-digitated transducer periodic structure in a synchronous, i.e.,with the same period and same metallization ratio, or in anon-synchronous way.

In another variant, the at least one of the reflecting structures 212,214, 216, 218 comprises more than one reflector, wherein the reflectorscan have the same number of metallic strips 220 or not.

In a variant of the present disclosure, the metallic strips 220 of thereflecting structures 212, 214, 216, 218 can be connected to each otherand/or shortened. This can result in an improvement of the reflectioncoefficient of the reflecting structures at the Bragg condition comparedto electrically isolated reflecting structures. At the Bragg condition,the reflected waves due to electrical and mechanical loading are inphase so that an improved reflection coefficient of the reflector at theBragg condition results in a better detection of the reflected waves bythe corresponding transducer structure.

The transducer structures 208 a, 208 b, and the transducer structures210 a, 210 b each comprise two inter-digitated comb electrodes 224 a,226 a, 224 b, 224 b and 240 a, 242 a, 240 b, 242 b. The comb electrodes224 a, 226 a, 224 b, 224 b and 240 a, 242 a, 240 b, 242 b are formed ofany suitable conductive metal, for example, Aluminum or Aluminum alloy.Nevertheless, other material may be used that generates strongerreflection coefficient for smaller electrode relative thickness. In thatmatter, the preferred electrode materials are Copper (Cu), Molybdenum(Mo), Nickel (Ni), Platinum (Pt) or Gold (Au) with an adhesion layersuch as Titanium (Ti) or Tantalum (Ta) or Chromium (Cr), Zirconium (Zr),Palladium (Pd), Iridium (Ir), Tungsten (W), etc. In FIG. 2a , theelectrodes comprise fingers. In a variant of the embodiment, they couldalso have spilt fingers comprising each two or more directly adjacentelectrode fingers belonging to the same comb electrode. In anothervariant, the electrode fingers can be slanted enabling a beam-steeringcompensation.

The transducer structures 208 a, 208 b and 210 a, 210 b are also definedby the electrode pitch p (not shown), corresponding to the edge-to-edgedistance between two neighboring electrode fingers from opposite combelectrodes 224 a, b and 226 a, b and 240 a,b and 242 a,b. In a variantof the present disclosure, the electrode pitch p is defined by the Braggcondition given by p=λ/2, λ being the operating acoustic wavelength ofthe transducer structures 212, 214. By operating acoustic wavelength λ,one understands X being the acoustic wavelength following λ=V/f with fthe predetermined central frequency of the resonator structure and V thephase velocity of the utilized mode. Such transducer structure, as shownin FIG. 2a , is also said to be a 2-finger-per-wavelengthinter-digitated transducer (IDT).

In a variant of the present disclosure, the inter-digitated transducerstructure 208, 210 can operate out of the Bragg conditions, forinstance, using a 3 or 4-finger-per-wavelength excitation structure or5-finger-per-two-wavelength transducers or 7 or 8 finger-per-threewavelength.

The transducer structures 208 a, 208 b and 210 a, 210 b can besymmetrical, namely they have the same number of electrode fingers withthe same characteristics. However, in a variant of the presentdisclosure, they can also be different, in particular, they can have adifferent number of electrode fingers and/or a different pitch p.

In a variant of the present disclosure, the inter-digitated transducerstructures 208 a, 208 b and 210 a, 210 b can be tapered to reducetransverse modes.

The substrate 206 over or in which the resonators 202, 204 are providedis a piezoelectric bulk material, with crystallographic axis X, Y and Zas shown in FIG. 1. The piezoelectric substrate 206 herein described byway of example may be Quartz, in particular, AT-cut Quartz.

According to a variant of the present disclosure, the acoustic wavepropagating substrate 206 on which the resonators 202, 204 and hence thetransducer structures 208 a, 208 b and 210 a, 210 b and the reflectingstructures 212, 214, 216, 218 are provided can be a composite substrate206. The composite substrate 206 comprises a layer of piezoelectricmaterial of a certain thickness, formed on top of a base substrate. Thepiezoelectric layer by way of example may be Lithium Tantalate (LiTaO3)or Lithium Niobate (LiNbO3).

According to the present disclosure, the resonators 202, 204 arepositioned on the substrate 206 so that they have two different surfaceacoustic wave propagation directions but due to cross like arrangementof the two resonators 202, 204, they are sharing at least partially thesame area on the substrate 206.

In this embodiment, the first resonator 202 is positioned so that itsdirection of propagation of acoustic wave is in the crystallographicdirection X of the acoustic wave propagating substrate 206. In FIG. 2a ,the direction of propagation of the acoustic wave of the secondresonator 204 is in the crystallographic direction Z of thepiezoelectric substrate 206. Thus, the propagation direction of theacoustic wave for the resonator 204 is rotated by an angle Θ=90°compared to the crystallographic direction X of the acoustic wavepropagating substrate 206, and compared to the surface acoustic wavepropagation direction of the first resonator 202. In FIG. 1a , the angleΘ has a value of 90°, but in a variant of the embodiment, the anglecould be different. In a variant, the angle Θ can be lower than 90°, forexample, with a variation of 10°, which enables correction of effectssuch as beam steering. In these variants, however, the symmetry with thecrystallographic axis X still remains in order to keep equal propertiesalong the two propagation directions.

In this embodiment, besides their wave propagation direction, theresonators 202, 204 have the same geometrical structure, meaning thattheir transducer structure 208 a, 208 b and 210 a, 210 b, respectively,and the reflecting structures 212, 214, 216, 218 have the same designsand/or dimensions. In a variant of the embodiment, they can have adifferent design, e.g., different dimensions and/or different geometry.For example, the reflecting structures 212, 214, 216, 218 can bedifferent but the transducer structures 208 a, 208 b and 210 a, 210 bare the same or vice-versa or both the reflecting structures 212, 214,216, 218 and the transducer structures 208 a, 208 b and 210 a, 210 b canbe different.

In this embodiment, the resonators 202, 204 are split into two parts,each part of a resonator being separated from the other part by acertain distance d1, d2, respectively.

The cavity 222 located in between the split parts 202 a, 202 b and 204a, 204 b of the two resonators 202, 204, with its dimensions defined bythe distances d1 and d2, corresponds to an acoustic cavity 222, inparticular, a resonant acoustic cavity 222. In FIG. 2a , the distancesd1 and d2 are identical, but in a variant of the embodiment, they can bedifferent.

In this embodiment, the two split parts 202 a, 202 b of the resonator202 are symmetrical in regards to the cavity 222 and identical to eachother so that the cavity 222 is actually located in the center part ofthe resonator 202. In a variant of the embodiment, the two split parts202 a, 202 b of the resonator 202 are not identical and/or symmetric inregards of the cavity 222.

In this embodiment, furthermore, the two split parts 204 a, 204 b of theresonator 204 are also symmetric in regards to the cavity 222 andidentical to each other so that the cavity 222 is actually located inthe center part of the resonator 204. Thus, in FIG. 2a , the cavity 222is a central cavity common to both resonators 202, 204.

In this embodiment, furthermore, the split parts 202 a, 202 b and 204 a,204 b of both resonators 202, 204 are symmetric in regards to the cavity222 and identical. In a variant of the embodiment, the split parts 202a, 202 b and 204 a, 204 b of the resonators 202, 204 are not identicaland/or symmetric in regards of the cavity 222.

The resonators 202, 204 are split in a manner so that actually, thetransducer structure of the resonator is split into two parts 208 a and208 b and 210 a and 210 b. Thus, each split part 202 a, 202 b, 204 a,204 b of the resonators 202, 204 actually comprises a reflectingstructure and a split part of the transducer structure of the respectiveresonator. Hence, the split part 202 a of the resonator 202 comprisesthe reflecting structure 212 and the split part 208 a of the transducerstructure. The split part 202 b of the resonator 202 comprises thereflecting structure 214 and the split part 208 b of the transducerstructure. The split part 204 a of the resonator 204 comprises thereflecting structure 216 and the split part 210 a of the transducerstructure. The split part 204 b of the resonator 204 comprises thereflecting structure 218 and the split part 210 b of the transducerstructure.

In a variant of the embodiment, the resonator is split in between onereflecting structure and the transducer structure. Thus, one split partof the two split parts of the resonator comprises the entire transducerstructure with one reflecting structure and the other part the otherreflecting structure.

FIG. 2b shows a surface acoustic wave device according to a secondembodiment of the present disclosure. Elements carrying the samereference numeral as in FIG. 2a will not be described again in detail,as they correspond to the ones already described above.

Unlike in the first embodiment, both resonators 202, 204 of the surfaceacoustic wave sensor 300 are now positioned at an angle Ψ to theacoustic propagation direction X of the piezoelectric substrate 306 incomparison with the surface acoustic wave sensor 200 of the firstembodiment. This is the only difference with respect to the firstembodiment.

Thus, the propagation direction of the acoustic wave for the resonator202 is rotated by an angle Ψ compared to the crystallographic directionX of the acoustic wave propagating substrate 306.

The resonator 204 is still positioned on the acoustic propagatingsubstrate 206 at an angle Θ=90°. In a variant of the embodiment, anothervalue of Θ different to 90°, for example, smaller than 90°, could beused, which would enable correction of effects such as beam steering.

FIG. 3a shows a third embodiment according to the present disclosurebased on the sensor 200 according to the first embodiment. In thisembodiment, the two resonators are electrically connected in adifferential manner, thereby forming a differential resonator device350. This configuration can be used to measure stress, e.g., due topresence of radial forces. Elements carrying the same reference numeralas in FIG. 2a will not be described again in detail, as they correspondto the ones already described above.

In this embodiment, the comb electrodes 224 a, 242 a, 226 b and 240 bare electrically connected by the conductive line 356 and the combelectrodes 224 b, 242 b, 226 a and 240 a are electrically connected bythe conductive line 358 to form a differential arrangement. Theresonators 202, 204 are here connected in parallel and the resonatordevice 350 operates at resonance.

In a variant of the present disclosure, the two resonators can beconnected in series and the resonator device would operate atanti-resonance operation.

The resonator device 350 according to the third embodiment allows toposition a test area in the central cavity shared by both resonators andto conduct a measure at the same location by the two resonators,yielding an improvement in the measurement quality and also a betterimmunity to parasitic stress effects compared to the state of the artdevice described with reference to FIG. 1.

In FIG. 3a , the two resonator propagation directions, shown as arrows352, 354, are respectively parallel and perpendicular to the directionof an applied radial force Fr yielding equal and opposed stresses forthe two resonators 202, 204. Here, the two principal strain componentsin the substrate 206 are aligned to the two principal strain componentsin the object due to external radial stress. The propagation directionsof the surface acoustic waves propagating through the respectiveresonators 202 and 204 may be respectively aligned with each of the twoprincipal strain components of the substrate 206 and the two principalstrain components of the object due to external stress. Thus, whenstress is applied, one of the resonators will be in tension and theother one will be in compression. As a result, their resonantfrequencies f1, f2 will change in opposite directions. By sensing thechange of the difference Δf between the two resonant frequencies, onecan find the applied torque M, as the difference Δf is linearlyproportional to the torque M.

Sensing of the change of the difference frequency Δf permits suppressionof a number of common-mode interference factors and, reduce variationsdue to a temperature, which should cancel out in the differentialsensing arrangement.

FIG. 3b shows the fourth embodiment according to the present disclosurewherein the SAW sensor 300 according to the second embodiment isconfigured to measure the stress due to tangential forces. In thisembodiment, the two resonators 202 and 204 are electrically connected ina differential manner, thereby forming a differential resonator device370. This configuration can be used to measure stress, e.g., due topresence of tangential forces. Elements carrying the same referencenumeral as in FIGS. 2a and 2b will not be described again in detail, asthey correspond to the ones already described above.

In this embodiment, the comb electrodes 224 a, 242 a, 226 b and 240 bare electrically connected by the conductive line 376 and the combelectrodes 224 b, 242 b, 226 a and 240 a are electrically connected bythe conductive line 378 to form a differential arrangement. Theresonators 202, 204 are here connected in parallel and the resonatordevice 370 operates at resonance.

In a variant of the present disclosure, the two resonators can beconnected in series and the resonator device would operate atanti-resonance operation.

The resonator device 370 according to the fourth embodiment allowspositioning of a test area in the central cavity shared by bothresonators and conduction of a measure at the same location by the tworesonators, yielding an improvement in the measurement quality and alsoa better immunity to parasitic stress effects compared to the state ofthe art device described with reference to FIG. 1.

In FIG. 3b , the two sensor propagation directions are shown in FIG. 3bas arrows 372 and 374. In the tangential mode, the stress is orthogonalto the radial direction. It must be considered that the tangential forceis exerted on the edge of the object, its central part being blocked.Therefore, due to the reaction of the fixed part, everything happenslike in the case of torque. Projecting the resulting force on the tworesonators yield one compressional effect for one resonator and oneextensional effect for the other one resonator, thus yielding adifferential mode.

The resonators 202, 204 of the SAW device 370 are laid down on apiezoelectric substrate 206 so that the surface acoustic waves propagateat an angle Ψ of 45° relative to the crystallographic axis X of thepiezoelectric substrate 206. At this angle, the contribution oftemperature variations of third order elastic constants of the substrate206 to the temperature variation of the Force sensitivity, issubstantially equal and opposite to the sum total of variations inlinear temperature coefficient of expansion, non-zero third orderelastic constants, temperature variation of contributions caused byfirst order elastic constants, and temperature variations of substratedensity. Thus, the resonator device 370 achieves a reduction oftangential force sensitivity variation with temperature.

The resonator device according to the present disclosure thus operatesas a differential sensor in differential mode to segregate the twoconsidered mechanical effects of radial, see FIG. 3a , and tangentialforce, see FIG. 3 b.

FIG. 3c shows the electrical admittance simulation of a surface acousticwave device according to the second embodiment of the presentdisclosure, as illustrated in FIG. 2b . For this simulation, a Quartzsubstrate with a (YXlt)/39_(O)/±45° cut was used. The aperture of bothresonators is 300 μm, the cavity length is 361 μm, the pitch in theinter-digitated transducer structure of the first resonator is 3.74 μmand 3.743 μm in the mirror, for the second resonator, these values are3.735 μm and 3.738 μm, respectively. To reduce spurious resonance on theresonator spectrum signature, small gaps (300 nm and 500 nm) between thetransducer structures and the associated mirrors were introduced. Themetal thickness (AlCu) is 245 nm. All the gratings operate at Braggconditions. Mirrors are composed of 300 strips and the resonators of 120(2×60) and 140 (2×70) finger pair for impedance matching and theresonators are slanted to compensate the beam steering (4°). The tworesonators of the surface acoustic wave device are positioned at anangle of 90° to each other, thus in a cross-type formation. Furthermore,the two resonators are positioned at an angle of 45° to the propagationdirection X of the quartz substrate. The resonators are identicallysplit into two parts, both parts being symmetrical to each other and acentral acoustic cavity shared by both resonators can be seen asdescribed in FIGS. 2a and 2 b.

The electrical admittance graph plots the conductance (S) and thesusceptance (S) on the right and left Y axis, respectively, in functionof the frequency (MHz) on the X axis. As two resonators are present, tworesonance peaks are visible in the electrical admittance graph, slightlyabove 434 MHz and slightly above 434.3 MHZ, respectively, for both thereal part of the admittance (conductance G) and the imaginary part ofthe admittance (susceptance B). The two resonance peaks are balanced toapproach a 50Ω matching within the 434 MHz centered ISM-band.

When a radial force is applied to the device, as in the third embodimentof the present disclosure as shown in FIG. 3a , the applied radial forceFr yields equal and opposed stresses for the two resonators 202, 204.Here, the two principal strain components in the substrate 206 arealigned to the two principal strain components in the object due to theapplied stress. The propagation directions of the surface acoustic wavespropagating through the respective resonators 202 and 204 arerespectively aligned with each of the two principal strain components ofthe substrate 206 and the two principal strain components of the objectdue to stress. Thus, when radial stress is applied, one of theresonators will be in tension and the other one will be in compression.As a result, their resonant frequencies f1, f2 will change in oppositedirections, as shown in FIG. 3c with the arrows of case a) and b). Thechange in Δf between the two resonant peaks is then proportional to theapplied force F.

In contrast to the state of the art, the resonators 202 and 204 have acommon cavity 222, which corresponds to the location where themeasurement is made for both resonators 202, 204. Thus, both resonatorswill measure at the same location and a more accurate value of theapplied force will be obtained, compared to the value obtained with astate of the art device as shown in FIG. 1 a.

FIG. 4a shows a sensing device according to a fifth embodiment of thepresent disclosure.

In FIG. 4a , the sensing device 400 comprises two differential sensors350 and 370 according to the third and fourth embodiment of the presentdisclosure, respectively.

In the embodiment of FIG. 4a , the differential sensors 350 and 370 areeach located on a quartz dice 402, 404. In a variant of the embodiment,the same quartz dice could comprise both differential sensors 350 and370. They are not described again in detail but reference is made totheir description above.

Both quartz dices 402, 404 are positioned on an object 406, in order tomeasure e.g., the stress generated by tangential and radial forces onthe object 406. In FIG. 4a , the object is a wheel. The quartz dice 402and 404 are positioned on the same radial line 408, the quartz dice 404closer to the center of the object 406 than the quartz dice 402. Thequartz dice position on the object 406 could also be swapped so that thequartz dice 402 is the one closer to the center of the object 406.

The quartz dices 402, 404 are glued onto the object 406, which comprisesa steel plate at that position, with cyano-acrylate glue (M-bond 200)but any other glue or solid state attachment techniques could be used.

The resonators 202, 204 are split into two parts, as described in thethird and fourth embodiment, so that the differential sensors 350 and370 each comprises a central cavity 222, shared by the two resonators202, 204 of each sensor 350, 370.

In this embodiment, the differential sensor 370 is configured to measurethe stress on the object 406 due to the tangential forces while theother differential sensor 350 is configured to measure the stress on theobject 406 due to radial forces as explained above.

Both differential sensors 350, 370 are connected to an antenna 410, totransmit the measurements. In a variant of the embodiment, eachdifferential sensor can have its own antenna.

According to the present disclosure, the stress resulting from theforces applied to the object 406 and sensed by the sensors 350, 370 ismeasured at the same location at the central cavity 222 for each sensor350, 370, yielding an improvement in the measurement quality and abetter immunity to parasitic stress effects.

In a variant, the sensing device 400 can comprise more than twodifferential sensors according to the present disclosure.

In another variant of the present disclosure, the sensing device 400 canbe applied to any other object, and not only a wheel, in order tomeasure concomitantly the stress due by the radial and tangential forcesexperienced by the object. Other physical parameters, outside of stress,can also be measured with the sensing device 400. For example, torsionaleffects and torque can also be measured or any other physical parameternot related to stress.

In another variant of the present disclosure, the sensing device 400 canmeasure the stress due by the radial and tangential forces experiencedby the object at the same location. The four resonators of the sensingdevice would share the same resonant cavity.

FIG. 4b shows the electrical admittance simulation of the sensing deviceaccording to the fifth embodiment of the present disclosure. The sensingdevice 400 as shown in FIG. 4a comprises two differential sensors 350,370, each comprising two resonators 202, 204. The differential sensor350 is according to the third embodiment as described in FIG. 3a and thedifferential sensor 370 is according to the fourth embodiment asdescribed in FIG. 3 b.

The electrical admittance graph plots the conductance (in Siemens—S) andthe susceptance (in S) on the right and left Y axis, respectively, infunction of the frequency (MHz) on the X axis. As two differentialsensors are present, each comprising two resonators, four resonancepeaks are visible in the electrical admittance graph, slightly above andbelow 434 MHz, for both the real part of the admittance (conductance G)and the imaginary part of the admittance (susceptance B). The resonancepeaks of each resonator are balanced to approach a 50Ω matching withinthe 434 MHz centered ISM-band.

FIGS. 5a to 5d and FIGS. 6a, 6b and FIGS. 7a to 7e show multiplevariants of the resonator device according to the present disclosure.

The basic structure corresponds to the one of the first embodiments andonly the differences with respect to that one will be described. Thus,the features common with the first embodiment of FIG. 2a will not bedescribed in detail again but reference is made to their descriptionabove. Furthermore, the variants will be shown based on the structure ofthe first embodiment but they can be applied to the structure of thesecond, third or fourth embodiments as well.

FIG. 5a shows a variant of the first embodiment of FIG. 2a , where thereflecting structures 512, 514, 516 and 518 of the resonators 502, 504comprises metallic strips 520, which are connected to each other and/orshortened. This results in an improvement of the reflection coefficientof the reflecting structures at the Bragg condition. At the Braggcondition, the reflected waves due to electrical and mechanical loadingare in phase so that an improved reflection coefficient of thereflecting structures 512, 514, 516 and 518 or at the Bragg conditionresults in a better detection of the reflected waves by thecorresponding transducer structure 208 a, 208 b and 210 a, 210 b.

The resonator device 500 as described in this variant enables managementof parasites due to directivity effects.

FIG. 5b shows a variant of the first embodiment where the split parts ofthe resonators 602, 604 are not identical or symmetrical with respect tothe cavity 622. In FIG. 5b , the split occurs between the transducerstructure 208, 210 and one of the reflecting structures 212, 216 of theresonator 602, 604, respectively. The split part 602 a of the resonator602 comprises only the reflecting structure 212. The split part 602 b ofthe resonator 602 comprises the entire transducer structure 208 and thereflecting structure 214. Correspondingly, the split part 604 a of theresonator 604 comprises only the reflecting structure 216. The splitpart 604 b of the resonator 604 comprises the entire transducerstructure 210 and the reflecting structure 218. The cavity 622 is notperfectly in the center of the two resonators 602, 604 but is stillshared by both resonators 602, 604.

The resonator device 600 as described in this variant enables managementof parasites due to directivity effects.

FIG. 5c shows a further variant of FIG. 6b , thus, of the firstembodiment, where the transducer structure 708, 710 of the resonators702,704 comprises electrode means in the form of split fingers 728. Thesplit fingers 728 comprise each two directly adjacent electrode fingersbelonging to the same comb electrode 724. Thus, the transducerstructures 708, 710 do not operate at the Bragg conditions.

Furthermore, the split parts of the resonators 702, 704 are alsodifferent and not symmetric with regards to the cavity 722, as thereflecting structures 712, 714 and 716, 718 are not identical within aresonator 702, 704, respectively. For the resonator 702, the reflectingstructure 714 comprises more metallic strips 120 as the reflectingstructure 712 (same thing for the resonator 704). The metallic strips120 are also connected to each other and/or shortened. In a variant,they can also not be connected to each other.

Here, like in the second variant of the first embodiment, the split partof a resonator comprises a reflecting structure alone and the othersplit part of the resonator comprises the full transducer structure andthe other reflecting structure adjacent the transducer structure. Again,the cavity 722 is not central within the resonators 702, 704, but isstill shared by the two resonators 702, 704.

The resonator device 700 as described in this variant enables managementof parasites due to directivity effects.

FIG. 5d shows a further variant of FIG. 5c , where the transducerstructures 708, 810 of the resonators 702, 804 are different. Theresonator 702 corresponds to the resonator of FIG. 5c , wherein thetransducer structure 708 comprised split fingers 728 as electrode meansand thus, does not work at the Bragg condition. In the contrary, theresonator 804 is the same as in FIG. 1a and the transducer structure 804works at the Bragg condition and is a 2-finger-per-wavelengthinter-digitated transducer (IDT). Again, like in FIG. 1a , the splitparts 804 a and 804 b of the resonator 804 each comprise a reflectingstructure 716, 818 and a part of the transducer structure 810 a, 810 b,respectively. While for the resonator 702, one split part 702 acomprises the reflecting structure 712 and the entire transducerstructure 708 and the other split part 702 b comprises only a reflectingstructure 714.

The metallic strips 120 of the reflecting structures 712, 714 and 716,818 are also connected to each other and/or shortened. In a variant,they can also not be connected to each other.

The resonator device 800 as described in this variant enables managementof parasites due to directivity effects.

FIG. 6a shows a variant of the first embodiment where the cavity 922 ofthe differential sensor 900 is metalized. The cavity 922 is a centralcavity, as shown in FIG. 1a . The metallization of the cavity 922 can bedone on the whole surface as shown in FIG. 6a , but it can also be doneonly on part of the surface of the cavity 922. Thus, the surface of thecavity 922 can be fully metalized or partially metalized.

Again, the metallic strips 120 of the reflecting structures 212, 214 and216, 218 are also connected to each other and/or shortened. In avariant, they can also not be connected to each other.

The resonator device 900 as described in this variant enables filteringor selection of the possible modes of the structure or even allowsoperation in a coupled mode configuration.

FIG. 6b shows the variant of the first embodiment where the cavity 1022of the differential sensor 1000 comprises one or more gratings 1024,1026. The grating 1024, 1026 can be a metal grating, deposited on top ofthe surface of the cavity 1022, or it can also be an etched grating.When a single grating is present, it can be a one direction grating.When multiple gratings are present, the gratings 1024, 1026 can besuperimposed within the surface of the cavity 1022, as shown in FIG. 6b. In a variant, the gratings 1024, 1026 can be located within the fullsurface of the cavity 1022 or only partially within the surface of thecavity 1022.

Again, the metallic strips 120 of the reflecting structures 212, 214 and216, 218 are also connected to each other and/or shortened. In avariant, they can also not be connected to each other.

The surface acoustic wave device 1000 as described in this variantenables filtering or selection of the modes of the structure or evenenables to operate in a coupled mode configuration.

FIG. 7a shows the surface acoustic wave differential sensor 1100according to a seventh variant of the first embodiment of the presentdisclosure.

In this variant, the reflecting structures of the resonators 1102, 1104comprise a plurality of reflectors, each comprising more or lessmetallic strips 1120. In this variant, the resonators 1102, 1104 are SAWtag devices. SAW tag devices are sensors, which can be remotelyinterrogated, providing a wireless measurement of a physical quantity.Whatever this physical quantity is, it is better to put in placedifferential measurement to guarantee the measurement of an absolutephysical quantity or to suppress correlated external perturbationsaffecting the sensor.

Two SAW-tags are used in a way that only the two first echoes are usedto determine the stress value, the other echoes may be used asidentification marks and/or as other physical effect markers (forinstance, temperature).

The SAW tag device 1102 comprises a transducer structure 1108, inparticular, only one transducer structure, and a set of reflectors 1114,1116 and 1118, positioned at various delays on one side of thetransducer structure 1108 in the direction of propagation X as shown inFIG. 7a . These reflectors 1114, 1116, and 1118 usually comprise one ormore metallic strips 1120, e.g., aluminum strips. The SAW tag device1102, 1104 also comprises an antenna (not shown) connected to thetransducer structure 1108, 1110.

The SAW tag device 1104 is the same as the SAW tag device 1102 but itsset of reflectors 1114, 1116, and 1118, positioned at various delays onone side of the transducer structure 1110 in the direction ofpropagation Y as shown in FIG. 7 a.

The SAW tag devices 1102, 1104 are actually split in two parts 1102 aand 1102 b, 1104 a and 1104 b, between the inter-digitated transducerstructures 1108, 1110 and the first reflector 1116, so that one part ofthe split SAW tag 1102 a, 1104 a comprises the set of reflectors 1114,1116, and 1118 or delay line and the other part 1102 b, 1104 b of theSAW tag devices 1102, 1104 comprises only the inter-digitated transducerstructure 1108, 1110.

The inter-digitated transducer structures 1108, 1110 are operating atBragg conditions but could operate out of this condition. The reflectors1114, 1116 and 1118 are in open circuit mode. The distances L11, L12,L13 and L21, L22 and L23 between the reflectors 1114, 1116 and 1118 andthe transducer structure 1108, 1110 are chosen such that thecorresponding echoes do not overlap over the whole measurement range.The cavity 1122 is shared by both resonators 1102, 1104, although thecavity 1122 is not centrally located between the two resonators 1102,1104, since the split parts 1102 a and 1102 b and the split parts 1104 aand 1104 b of both resonators 1102, 1104 are not the same and notsymmetric to each other.

In the variant shown in FIG. 7b , the split part 1102 b and 1104 bcomprises a reflecting structure 1118 and the inter-digitated transducerstructure 1108, 1110, respectively. The inter-digitated transducerstructures 1108, 1110 are operating at Bragg conditions but couldoperate out of this condition, they are associated here with areflecting structure 1118 to reflect and launch all the energy towardthe obstacle, the mirrors on which the waves partially reflects areshorten and both delay lines are identical. The distances L11, L12, L13and L21, L22 and L23 between the reflectors 1114, 1116, 1118 and thetransducer structures 1108 and 1110 are chosen such that thecorresponding echoes do not overlap over the whole measurement range.

In the variant in FIG. 7c , the surface acoustic wave device 1300 is avariant of the device 1200 of FIG. 7b , where the split part 1204 a ofthe resonator 1204 only comprises one reflecting structure 114 and thesplit part 1202 a of the resonator 1202 comprises more reflectors thanthe split part 1102 a of the device 1200. In this variant, bothresonators 1202 and 1204 are different, although both are SAW tagdevices. The resonator 1204 or SAW tag 1204 comprises a lot morereflectors as the SAW tag 1204.

Again, the cavity 1222 is shared by both resonators 1202, 1204, althoughthe cavity 1222 is not centrally located between the two resonators1202, 1204, since the split parts 1202 a and b and the split parts 1204a and b of both resonators 1202, 1204 are not the same and not symmetricto each other.

The metallic strips 1120 of the reflectors 1118, 1116, and 1114 are alsoconnected to each other and/or shortened. In a variant, they can alsonot be connected to each other.

The resonator device 1500 as described in this variant measures thestress at the cavity 1422, which is located within the firsttransducer-reflector gap of the longest SAW-tag 1202. The cavity 1422 isdefined by the gap L11 and L21. In another variant, one SAW tag canshare more than one cavity with the other SAW tags or resonators. Thiswould enable measurement of a distribution of stress.

FIG. 7d is a variant of the resonator device 1300 of FIG. 7c , where thecavity 1222 is metallized. The metallization of the cavity 1222 can bedone on the whole surface as shown in FIG. 7d , but it can also be doneonly on part of the surface of the cavity 1222. Thus, the surface of thecavity 1222 can be fully metalized or partially metalized. In a variant,the cavity 1222 can also comprise a metallic grating or more than onemetallic grating superimposed to each other. In another variant, thecavity surface can be partially or fully covered by an active layer. Forexample, the active layer could be sensitive to a magnetic field.Therefore, by magnetostriction, the film may experience stresses thatcan be detected according to the present disclosure. The active layercould also be a layer that changes its properties when exposed to gas,e.g., palladium and hydrogen.

The resonator device 1400 as described in this variant enablesincreasing the sensor sensitivity or more generally optimization of thesensor operation.

FIG. 7e is a variant of the resonator device 1300 of FIG. 7c , where thesplitting of the resonator 1202 is done in between the delay lines orreflectors 1114, 1116 and 1118, such that the split part 1202 b of theresonator 1202 comprises now some reflectors 1114, 1116 and 1118 on oneside of the transducer structure 1108 with the transducer structure 1108and the reflecting structure 1118 on the other side of the transducerstructure 1108. The resonator 1204 is the same as in FIG. 7 c.

The resonator device 1500 as described in this variant measures thestress at the cavity 1422, which is located anywhere else on the delayline of the resonators but between the first transducer-reflector gap,defined by the distance L11 and L21, of the longest SAW-tag 1202.

A number of embodiments of the present disclosure have been described.Nevertheless, it is understood that various modifications andenhancements may be made without departing from the scope of theinvention as defined by the following claims.

1.-17. (canceled)
 18. A resonator device for measuring stress,comprising: at least two resonators, each resonator comprising aninter-digitated transducer structure arranged between two reflectingstructures on or in a piezoelectric substrate; wherein the at least tworesonators are arranged and positioned such that they have two differentwave propagation directions; and wherein each resonator comprises atleast two parts with an area between the two parts of the at least tworesonators forming a cavity, wherein the cavity is shared by the atleast two resonators.
 19. The resonator device of claim 18, wherein eachof the at least two parts of the at least two resonators comprises atleast one reflecting structure and a part of the inter-digitatedtransducer structure of the corresponding resonator.
 20. The resonatordevice of claim 18, wherein the inter-digitated transducer structure ofthe resonator comprises inter-digitated comb electrodes, and wherein,for at least one transducer structure of the at least two resonators,the inter-digitated comb electrodes are defined by the Bragg conditiongiven by p=λ/2, λ being the operating acoustic wavelength of thetransducer structure and p being an electrode pitch of the transducerstructure.
 21. The resonator device of claim 18, wherein the twodifferent wave propagation directions form an angle Θ with each other, Θbeing equal to ±90° or smaller.
 22. The resonator device of claim 18,wherein electrodes of the inter-digitated transducer structure of oneresonator of the at least two resonators are electrically connected withelectrodes of the transducer structure of another resonator of the atleast two resonators in a differential manner.
 23. The resonator deviceof claim 18, wherein at least one of the resonators is arranged andpositioned such that its wave propagation direction is parallel to acrystalline axis of the piezoelectric substrate.
 24. The resonatordevice of claim 18, wherein at least one of the resonators is arrangedand positioned such that its wave propagation direction makes an angle Ψto one of the crystalline axis of the piezoelectric substrate.
 25. Theresonator device of claim 24, wherein the angle Ψ is equal to ±45°. 26.The resonator device of claim 18, wherein at least a part of a surfacedefining the cavity is metalized.
 27. The resonator device according toclaim 26, wherein the metallization of the cavity comprises at least onegrating.
 28. The resonator device of claim 18, wherein each one of thereflecting structures of the resonators comprises one or more metallicstrips, the metallic strips being connected to each other or connectedto ground.
 29. The resonator device of claim 18, wherein each resonatorof the at least two resonators is a surface acoustic wave resonator, abulk acoustic wave resonator, a Love wave or a Lamb wave or shear-platemode acoustic resonator.
 30. A differential sensing device comprising atleast one resonator device according to claim
 18. 31. The differentialsensing device of claim 30, wherein the propagation direction of one ofthe resonators is parallel or perpendicular to a radial direction tosense a radial force.
 32. The differential sensing device of claim 30,wherein the propagation direction of one of the resonators is at anangle with respect to a radial direction to sense a tangential force.33. The differential sensing device of claim 32, wherein the angle isequal to ±45°.
 34. The differential sensing device of claim 30, furthercomprising an antenna connected to the at least one resonator device.35. The differential sensing device of claim 30, wherein the at leasttwo resonators are provided on the same piezoelectric substrate.
 36. Adifferential sensing device comprising at least one resonator deviceaccording to claim 18, wherein the propagation direction of one of theresonators is parallel or perpendicular to a radial direction to sense aradial force; and wherein the propagation direction of another of theresonators is at an angle with respect to a radial direction to sense atangential force.
 37. The differential sensing device of claim 36,wherein the angle is equal to ±45°
 38. The differential sensing deviceof claim 36, wherein the at least two resonators are provided on thesame piezoelectric substrate.